Proton-ionizable Crown Ethers Research Articles

Synthesis, experimental and theoretical studies on the factors influencing the pKa values of new crown ethers containing a diarylphosphinic acid unit

Synthesis of acidic new crown ethers containing a diarylphosphinic acid unit has been accomplished. The aromatic rings of the crown ethers were substituted with tert-butyl and nitro groups. Nitro substitution of the crown ethers was investigated. pKa determination of the new proton-ionizable crown ethers has been performed, showing the effect of the substituents of the aromatic rings on the acidity. An anomaly was discovered in the pKa values and an explanation was given based on quantum mechanical calculations and molecular dynamics simulations.

Synthesis and molecular recognition studies of crown ethers

This short review summarizes the synthesis and molecular recognition studies of crown ether type macrocycles accomplished at the Institute for Organic Chemistry of Budapest University of Technology and Economics in the last few years. The research work reported here belongs to the areas of proton-ionizable crown ethers and chiral macrocycles. Proton-ionizable crown ethers at higher pHs than their pK_a values are mostly ionized to ligand anions which increase the cation-ligand complex stability with enhancement of selectivity and avoid the need for a counter anion to accompany the cation transport or solvent extraction. The latter factor is not only advantageous from energetical point of view, but is also important when counter anions are not required to be transported. Enantiopure chiral macrocycles have also drawn the attention of many researchers, because owing to their enantioselective complexation they are excellent candidates for effective sensors and selectors of the enantiomers of biologically important chiral compounds such as protonated primary organic amines, amino acids and the derivatives of the latters.

Synthesis of new enantiopure proton-ionizable crown ethers containing a dialkylhydrogenphosphate moiety

Seven new enantiopure proton-ionizable crown ethers containing a dialkylhydrogenphosphate moiety were prepared starting from optically active dialkyl-substituted oligoethylene glycols and phosphorus oxychloride followed by mild hydrolysis of the resulting macrocyclic chlorophosphates. Pentaethylene glycols having primary hydroxyl groups gave good yields of 17-crown-6 type ethers. Pentaethylene glycols with secondary hydroxyl groups rendered about the same amount of 17-crown-6 ethers and open chain dihydrogenphosphates in low yields. Tetraethylene glycols are reluctant to undergo macrocyclization with phosphorus oxychloride, especially the ones which contain secondary hydroxyl groups.

Synthesis of new proton‐ionizable crown ether compounds containing substituted lh‐pyridin‐4‐one subcyclic units

AbstractFive novel pyridono‐18‐crown‐6 (10‐14) and two new benzyloxy‐substituted pyridino‐18‐crown‐6 (15 and 16) ligands have been prepared. By the catalytic hydrogenative removal of the benzyl group from the benzyloxy moiety at position 4 of the pyridine ring of 15 and 16, pyridono‐18‐crown‐6 ethers 5 and 12 were obtained. These ligands were transformed to their 3,5‐dibromo (10 and 13) and 3,5‐dinitro derivatives (11 and 14) by treatment with bromine in methylene chloride and nitric acid in acetic anhydride, respectively. The latter proton‐ionizable crown ethers have pKavalues of about 7.5 for 10 and 13 and 4.5 for 11 and 14. Thus, they are good candidates for complexation and proton‐coupled transport of selected cations.

FLOTATION OF Co(II), Sr(II), AND Cs(I) CATIONS WITH PROTON-IONIZABLE LARIAT ETHERS

An experimental investigation is presented on flotation of Co(II), Sr(II), and Cs(I) cations from very dilute aqueous solutions with proton-ionizable crown ethers: monoethyl sym-(butyl)dibenzo-16-crown-5-oxymethylphosphonic acid, sodium 3-[sym-(decyl) dibenzo-16-crown-5-oxy]propanesulfonate, and sym-(decyl) dibenzo-16-crown-5-oxyacetic acid, used separately or together with a nonionic foaming agent (Triton X-100). The effect of major process variables on both the efficiency and the rate of flotation of Sr(II) cations with sym-(decyl)dibenzo- 16-crown-5-oxyacetic acid and Triton X-100 were established. Moreover, the competitive Sr2+/Cs+ flotation was studied with sulfonic and carboxylic 16-crown-5 derivatives. The carboxylic collector exhibited strong affinity towards Sr(II) cations and up to 1000-fold molar excess of lithium, ammonium, and potassium chlorides, and nitrates did not stop flotation of Sr(II) cations. Such affinity allowed for complete separation of Sr2+ from Cs+ in a single-stage batch flotation. Selectivity of flotation of Sr(II), Cs(I), and Co(II) cations with the sulfonic and carboxylic collectors did not follow the “crown ether cavity size” selectivity rule. Nevertheless, the separation of Sr and/or both Sr with Cs from extremely dilute aqueous solutions with these collectors was efficient and fast, which might have a practical meaning for the decontamination of aqueous solutions containing radioactive isotopes. Proton-ionizable crown ether derivatives of sufficient surface activity and water solubility could make a new generation of specific collectors for flotation of aqueous ions.

Sorption of lead(II) by proton-ionizable polyether resins

Novel ion-exchange resins have been prepared by condensation polymerization with formaldehyde of dibeozo acyclic and cyclic polyethers bearing a pendent acidic function (carboxylic acid, phosphonic acid monoethyl ester, phosphonic acid, or sulfonic acid). Such resins possess both ion-exchange and polyether-binding sites for metal-ion complexation. The efficiency of Ph 2+ sorption and selectivity relative to Zn 2+ sorption are assessed as a function of: (1) the acyclic, pseudocyclic, or cyclic nature of the polyether unit; (2) the identity of the proton-ionizable group; (3) the length of the spacer which joins the proton-ionizable group to the polyether framework; and (4) for the proton-ionizable crown ether resins, the atom or group which is geminal to the functional side arm.

Selective extraction of mercury with ionizable crown ethers in supercritical carbon dioxide

The solubility of a tert-butyl-substituted dibenzobistriazolo-crown ether in methanol (5 mol %) modified CO[sub 2] has been determined to be in the order of 1 x 10[sup [minus]3] M at 60[degree]C and 200 atm. Using this proton-ionizable crown ether as an extractant in methanol (5%) modified CO[sub 2], quantitative extraction of Hg[sup 2[minus]] from sand and cellulose-based filter papers can be achieved at 60[degree]C and 200 atm if a small amount of water is present in the solid matrix. Other divalent metal ions including Cd[sup 2[minus]], Co[sup 2], Mn[sup 2+], Ni[sup 2+], Pb[sup 2[minus]], and Zn[sup 2+] are virtually not extractable under these conditions. Gold (Au[sup 3[minus]]) can be extracted by the macrocyclic ligand with a lower efficiency than Hg[sup 2[minus]]. Extraction of Hg[sup 2[minus]] from aqueous samples can also be achieved using methanol-modified CO[sub 2] containing the macrocyclic ligand. Formation of extractable metal chelates with the ligand at the pH defined by the CO[sub 2]-H[sub 2]O system appears to be an important factor determining the extraction efficiencies of the metal ions by this in situ chelation-supercritical fluid extraction technique. 15 refs., 2 figs., 3 tabs.

Metal ion separations with proton-ionizable crown ethers and their polymers

Abstract Lipophilic crown ethers with pendent proton-ionizable groups are novel complexing agents for use in metal ion separations by solvent extraction. For a series of structurally related, lipophilic dibenzocrown ether carboxylic acids, the efficiency and selectivity of competitive alkali metal cation extraction for aqueous solution into chloroform is found to be strongly influenced by the crown ether ring size and the lipophilic group attachment site. Reaction of dibenzocrown ether carboxylic acids with formaldehyde in formic acid produces condensation polymers which possess both ion-exchange and cyclic polyether binding sites for metal ion complexation. These resins exhibit excellent exchange kinetics for competitive alkali metal cation sorption from aqueous solution and subsequent stripping and may be used in concentrator columns for the recovery of these metal ions from very dilute aqueous solution. Cation selectivity in the sorption and stripping steps is controlled by the structure of the crown e...

Selectivity in Stripping of Alkali-Metal Cations from Crown Ether Carboxylate Complexes

Abstract The potential of crown ethers (macrocyclic polyethers) as the next generation of specific extracting agents for metal ions was markedly enhanced by the introduction of crown ethers which bear pendent proton-ionizable groups (1–5). It has been shown that metal ion extraction does not require concomitant transfer of an aqueous phase anion into the organic medium (6). For potential practical applications of these proton-ionizable crown ethers in which hard aqueous phase anions (chloride, nitrate, sulfate) would be involved, this factor is of immense importance. In addition, the combination of ion binding cavities possessing fixed dimensions with protonionizable groups creates novel bifunctional chelating agents. A final factor is the case with which extracted metal ions may be stripped from the organic phase by shaking with aqueous mineral acid.

ChemInform Abstract: Proton‐Ionizable Crown Ethers. A Short Review

Abstract : Since Pederson reported the synthesis and cation-complexing characteristics of the crown ethers in 1967, there has been increasing interest in these compounds as complexing agents for various metal and organic cations. Many different modifications of the crowns have been made to enhance their cation-complexing properties. Some of these modifications involve the use of alkyl substituents, aromatic subcyclic units, nitrogen and/or sulfur atoms substituted for oxygen in the macroring and other changes which provide crowns with unique complexing properties. Macrocyclic crown ether ligands containing proton-ionizable groups are of interest because they alleviate the need for a counter anion in metal cation transport through a liquid membrane or in solvent extraction experiments. Often the cation-crown complex stability is increased when the crown is ionized. Transport of cations is, in many cases, also tied to the pH of the aqueous solutions in the liquid membrane system so that transport of cations can, in effect, be turned on and off by adjusting the pH. Proton- ionizable crown ligands have also been used spectrophotometrically to determine parts per million quantities of alkaline earth metal cations in solution. Having macrocycles which show selectivity in complexation and transport of cations is also an objective in the syntheses of these types of crown ligands. This short review summarizes the syntheses of proton-ionizable crown ether ligands (OICE) to the end of 1988, gives a listing of these compounds and briefly reports on some of their uses.

Proton-Ionizable Crown Ethers. A Short Review

Abstract : Since Pederson reported the synthesis and cation-complexing characteristics of the crown ethers in 1967, there has been increasing interest in these compounds as complexing agents for various metal and organic cations. Many different modifications of the crowns have been made to enhance their cation-complexing properties. Some of these modifications involve the use of alkyl substituents, aromatic subcyclic units, nitrogen and/or sulfur atoms substituted for oxygen in the macroring and other changes which provide crowns with unique complexing properties. Macrocyclic crown ether ligands containing proton-ionizable groups are of interest because they alleviate the need for a counter anion in metal cation transport through a liquid membrane or in solvent extraction experiments. Often the cation-crown complex stability is increased when the crown is ionized. Transport of cations is, in many cases, also tied to the pH of the aqueous solutions in the liquid membrane system so that transport of cations can, in effect, be turned on and off by adjusting the pH. Proton- ionizable crown ligands have also been used spectrophotometrically to determine parts per million quantities of alkaline earth metal cations in solution. Having macrocycles which show selectivity in complexation and transport of cations is also an objective in the syntheses of these types of crown ligands. This short review summarizes the syntheses of proton-ionizable crown ether ligands (OICE) to the end of 1988, gives a listing of these compounds and briefly reports on some of their uses.

Solvent extraction and bulk liquid membrane transport of Co(II) and Ni(II) ammine cations by proton-ionizable crown ethers

Separation efficiencies and selectivities in solvent extraction of Co(II) and Ni(H) ammine cations from aqueous solutions into chloroform and toluene and in transport through bulk toluene membranes by proton-ionizable crown ethers have been determined. Six proton-ionizable crown ethers with differing lipophilicities, polyether cavity sizes and ionizable groups (carboxylic and sulfonic acid functions) were examined. Higher selectivity and efficiency for Co(II) ammine cation extraction was observed for the more lipophilic, proton-ionizable crown ethers. Highly lipophilic crown carboxylic acids provided effective and selective transport of Co(II) ammine cations through bulk toluene membranes.

Comparative evaluation of neutral, and proton-ionizable crown ether compounds as lithium ionophores in ion-selective electrodes and in solvent extraction

Comparative evaluation of neutral, and proton-ionizable crown ether compounds as lithium ionophores in ion-selective electrodes and in solvent extraction

Extraction of Co(II) and Ni(II) ammine cations from aqueous solutions into chloroform by highly lipophilic crown carboxylic acids

The proton-ionizable crown ethers, 2-[(sym-dibenzo-14-crown-4)oxy]-decanoic acid (1), 2-[sym-dibenzo-16-crown-5)oxy]decanoic acid (2), and 2-[sym-dibenzo-19-crown-6)oxy]decanoic acid (3) efficiently extract Co(II) and Ni(II) ammine cations from highly alkaline aqueous solutions (pH>10) into chloroform. For extractions of the individual metal species,2 is more efficient than1 or3. In competitive extraction systems, good selectivity for Co(II) ammine cations over Ni(II) ammine cations is observed.